The problem of Web ratings

Today, Comscore confessed that its numbers for last fall are wrong. The principal source of the problem was its estimate of at-work use. At-work use is notoriously difficult to measure.
But the Times hints at a bigger problem with web site ratings: what are they for?
When advertisers are paying by the exposure, or even the click or the sale, is it really important which site is number one?
When advertisers are (properly) more focused on response than branding, and when its so easy to test a campaign on the Net, is it really important what the demographics of a site are?
The Times tells us some alarming things about differences between the services:

Comscore has cut its estimate of the time users spend on some sites by as much as 75%